Effects of cytochrome P450 (CYP) inducers and inhibitors on ondansetron pharmacokinetics in rats: involvement of hepatic CYP2D subfamily and 3A1/2 in ondansetron metabolism

Potential implications of DMET ontogeny on the disposition of commonly prescribed drugs in neonatal and pediatric intensive care units

Introduction: Pediatric patients, especially neonates and infants, are more susceptible to adverse drug events as compared to adults. In particular, immature small molecule drug metabolism and excretion can result in higher incidences of pediatric toxicity than adults if the pediatric dose is not adjusted.Area covered: We reviewed the top 29 small molecule drugs prescribed in neonatal and pediatric intensive care units and compiled the mechanisms of their metabolism and excretion. The ontogeny of Phase I and II drug metabolizing enzymes and transporters (DMETs), particularly relevant to these drugs, are summarized. The potential effects of DMET ontogeny on the metabolism and excretion of the top pediatric drugs were predicted. The current regulatory requirements and recommendations regarding safe and effective use of drugs in children are discussed. A few representative examples of the use of ontogeny-informed physiologically based pharmacokinetic (PBPK) models are highlighted.Expert opinion: Empirical prediction of pediatric drug dosing based on body weight or body-surface area from the adult parameters can be inaccurate because DMETs are not mature in children and the age-dependent maturation of these proteins is different. Ontogeny-informed-PBPK modeling provides a better alternative to predict the pharmacokinetics of drugs in children.

Identification of cytochrome P450s involved in the metabolism of arachidonic acid in human platelets

Although cytochrome P450s (CYPs) have been identified in most human cells, identification of CYPs in human platelets remains poorly explored. CYP expressions in human platelets were screened by using reverse transcriptase-polymerase chain reaction and western blot analysis followed by functional assays using arachidonic acid (ARA). CYP1A1, 2U1, 2J2, 4A11, 4F2, and 5A1 were expressed as both proteins and mRNAs in platelets. Ethoxyresorufin-O-deethylase activity was observed in platelets and this activity was significantly decreased after treatment with the general P450 inhibitor SKF-525A and the CYP1A inhibitor, α-naphthoflavone (40-45%, P<0.001). Seventeen ARA metabolites were detected in ARA-treated platelets. Among these, the levels of 20-hydroxyeicosatetraenoic acid and epoxyeicosatrienoic acids were significantly decreased with the treatment of the P450 ω-hydroxylase inhibitor 17-octadecynoic acid (P<0.05-0.001). In summary, multiple ARA-metabolizing P450s were identified in human platelets. These findings may provide an important resource for understanding physiological function of platelet.

Expression of arachidonic acid-metabolizing cytochrome P450s in human megakaryocytic Dami cells

Cytochrome P450s (P450s) are involved in the metabolism of arachidonic acid (ARA), and ARA metabolites are associated with various cellular signaling pathways, such as blood hemostasis and inflammation. The present study demonstrates the expression of ARA-metabolizing P450s in the human megakaryocytic Dami cells using reverse transcriptase-polymerase chain reaction (RT-PCR) and immunublotting analysis followed by activity assays using ARA as a substrate. In addition to the previously identified CYP5A1, both protein and mRNAs of CYP1A1, 2U1, and 2J2 bands were detected. Ethoxyresorufin-O-deethylase (EROD) activity was observed in Dami cells, and its activity was significantly decreased after treatment with the P450 inhibitor SKF-525A when compared to the control groups (60% reduction, P < 0.001). CYP1A1 protein expression in Dami cells was induced by 3-methylenecholantheren. This increase in CYP1A1 protein level was correlated with enhanced EROD activity (fourfold increase vs. the control), as well as with increased metabolites, such as 20-hydroxyeicosatrienoic acid (20-HETE), 14, 15-EET (14-,15-epoxyeicosatrienoic acid), and 14, 15-dihydroxyeicosatrienoic acid (14, 15-DHET). The expression of soluble epoxide hydrolase, an enzyme responsible for the synthesis of DHETs from EETs, was confirmed by RT-PCR. Furthermore, 15 ARA metabolites, including 8,9-EET, 14,15-EET, and 20-HETE, were detected by LC-MS/MS in ARA-treated Dami cells, and their levels were decreased with the treatment of the SKF-525A. The present data suggest the possibility that the P450s play a role in the metabolism of ARA and other CYP-related substrates in human megakaryocytes and that P450 expression in megakaryocytic cell lines may predict their existences in platelets with functional activities.

Pharmacokinetic interaction between tamoxifen and ondansetron in rats: non-competitive (hepatic) and competitive (intestinal) inhibition of tamoxifen metabolism by ondansetron via CYP2D subfamily and 3A1/2

PURPOSE

Tamoxifen and ondansetron were commonly metabolized via rat hepatic CYP2D subfamily and 3A1/2, and ondansetron is used to treat chemotherapy-induced nausea. The purpose of this study was to report the pharmacokinetic interaction between tamoxifen and ondansetron in rats.

METHODS

The pharmacokinetics of tamoxifen and ondansetron were evaluated after the intravenous and oral administration of tamoxifen, ondansetron, and both drugs together to rats. The Vmax (maximum velocity), Km (apparent Michaelis-Menten constant), CLint (intrinsic clearance), Ki (inhibition constant), and [I] (concentration of inhibitor in the liver and intestine)/Ki ratio of ondansetron were also measured.

RESULTS

The AUC0-infinitys of tamoxifen were significantly greater after both intravenous and oral administration with ondansetron compared to those of tamoxifen alone. The significantly slower hepatic and intestinal CLints for the disappearance of tamoxifen with both drugs together were due to inhibition of metabolism of tamoxifen by ondansetron via CYP2D subfamily and 3A1/2.

CONCLUSIONS

The significantly greater AUC0-infinity of tamoxifen after the intravenous administration of both drugs together could have possibly been attributable to a non-competitive (hepatic) inhibition of CYP2D subfamily- and 3A1/2-mediated tamoxifen metabolism by ondansetron. The significantly greater AUC0-infinity of tamoxifen after the oral administration of both drugs together could have been attributable to a competitive (intestinal) inhibition of CYP2D subfamily- and 3A1/2-mediated tamoxifen metabolism by ondansetron in addition to non-competitive inhibition in the liver.